High-Sensitivity Optical-Fiber Microfluidic Chip Based on Surface Plasmon Resonance
Objective Surface plasmon resonance (SPR) sensors based on Kretschmann prism are bulky and need to be equipped with mechanical movable parts, which is not conducive to the miniaturization and remote sensing of devices. Optical-fiber-based SPR technology has been widely used in food safety, environmental monitoring, and other fields owing to its label-free simple pretreatment, fast analysis, high sensitivity, and other excellent characteristics. However, most SPR sensors do not consider sample consumption in the process of sensor monitoring. The exposed optical fiber is fragile, which easily affects the sensor''s stability. Highly sensitive optical fibers, such as tapered and D-shaped fibers, are vulnerable and unstable. In addition to the abovementioned problems, the sensitivity of existing SPR fiber sensors needs further improvement. As a high-throughput microscale analysis device, the microfluidic chip system has shown great potential in remote monitoring, biological detection, and other fields in recent years. In this paper, a microfluidic chip based on SPR fiber sensor is designed and used to measure the concentration of a solution. This chip has the advantages of small size, compact structure, and low sample consumption; it also reduces the fiber damage and effectively improves the sensing stability by embedding the sensing fiber into Polydimethylsiloxane (PDMS) substrate. Photonic crystal fiber (PCF) is a new type of optical fiber. PCF comprises a single dielectric, in which air holes are closely arranged in the two-dimensional direction but unchanged in the axial structure to form microstructure cladding. Moreover, it can stimulate a more substantial SPR effect for its unique light-guiding characteristics.
Methods In this paper, PDMS—which has good chemical inertia and good biocompatibility—is used as the main material to fabricate the microfluidic chip. After cooling and forming in 3D mold, PDMS is stably bonded with glass sheet to form the main structure of the chip. The microchip contains a microfluidic channel (diameter: 200nm), whose length is the same as that of the chip. A sensing structure of multimode fiber-photonic crystal fiber-multimode fiber (MMF-PCF-MMF), which is coated with 60-nm gold film on the surface, is embedded in the channel to stimulate SPR effect. Then, mixed solutions of glucose solid sample and deionized water in a certain proportion with refractive index of 1.338--1.425 are used as samples to be tested. The experiment is performed at room temperature (25 °C). The glucose solution in the syringe is injected into the microfluidic chip using a syringe pump at a constant speed; the glucose solution flows through the optical fiber sensing part. When the concentration of the solution injected into the microfluidic chip is changed, the light wave meeting the resonance conditions excites the gold film to produce surface plasmon resonance and the resonant valley is generated on the transmitted spectrum. The position of the resonant valley shifts in the samples with different refractive indexes. By recording the resonance spectra of a series of samples, the sensitivity of the sensor with respect to the change in refractive index can be calibrated.
Results and Discussions When the refractive index of the liquid injected into the microfluidic chip increases from 1.338 to 1.425, the resonant wavelength of SPR spectrum shifts to longer wavelength because of the SPR resonance effect. The relationship between the refractive index and resonant wavelength of the liquid to be measured is extracted, and the sensitivity curve can be obtained by taking the resonant wavelength at n=1.338 as the starting point. The increasing speed of the sensitivity curve is related to the chip sensitivity. The experimental results show that the refractive index sensitivity of PCF SPR sensors can reach up to 8240.6nm/RIU, in which RIU is refractive index unit, thereby showing that these have good high-sensitivity characteristics and meet the application requirements of high sensitivity.
Conclusions In this paper, a novel microfluidic chip embedded with SPR sensor is designed and manufactured by combining the optical fiber structure, SPR effect, and microfluidic system. The volume of chip is approximately 3.5cm×1cm×5cm, which is much smaller than that of the traditional measurement instrument and is conducive to the integration of sensor. After testing the MMF-PCF-MMF structure based on SPR effect, the sensor sensitivity obtained is high (up to 8240.6nm/RIU) in a wide refractive index measurement range of 1.338--1.425. The proposed structure has good refractive sensitivity, small size, acid resistance, and corrosion resistance; these characteristics enable a broader application prospect in the field of biochemistry.
李致远：华中科技大学光电信息与工程学院, 湖北 武汉 430074
李正冉：华中科技大学光电信息与工程学院, 湖北 武汉 430074
王锦民：华中科技大学光电信息与工程学院, 湖北 武汉 430074
夏历：华中科技大学光电信息与工程学院, 湖北 武汉 430074
杨曌：华中科技大学光电信息与工程学院, 湖北 武汉 430074
李微：华中科技大学光电信息与工程学院, 湖北 武汉 430074
【1】Sun D F, Liu P. Solution concentration measurement system based on optical fiber spectrometer [J]. Physical Experiment of College. 2019, 32(3): 82-85.
孙德藩, 刘鹏. 基于光纤光谱仪的溶液浓度测量系统 [J]. 大学物理实验. 2019, 32(3): 82-85.Sun D F, Liu P. Solution concentration measurement system based on optical fiber spectrometer [J]. Physical Experiment of College. 2019, 32(3): 82-85.
孙德藩, 刘鹏. 基于光纤光谱仪的溶液浓度测量系统 [J]. 大学物理实验. 2019, 32(3): 82-85.
【2】Wang Q. The preparation and properties of optical fiber sensor based on surface plasmon resonance [D]. Nanjing: Nanjing University of Posts and Telecommunications. 2019, 1-3.
王旗. 基于光纤表面等离子共振传感器的制备及性能研究 [D]. 南京: 南京邮电大学. 2019, 1-3.
【3】Zhao J, Wang Y, Wang Y P. Graphene-oxide-enhanced surface plasmon resonance fiber sensor [J]. Laser & Optoelectronics Progress. 2019, 56(23): 230601.
赵静, 王英, 王义平. 氧化石墨烯增强型表面等离子体共振光纤传感器 [J]. 激光与光电子学进展. 2019, 56(23): 230601.
【4】Gao L, Gao W Z, Luo Z C, et al. SPR principle refractive index testing system for offshore oil spill [J]. Infrared and Laser Engineering. 2019, 48(8): 0813006.
高璐, 高文智, 罗政纯, 等. 面向海上溢油的SPR原理折射率检测实验系统 [J]. 红外与激光工程. 2019, 48(8): 0813006.
【5】Cai K J. Research on surface plasmon resonance refractive index sensor based on D-type optical fiber [D]. Nanjing: Nanjing University of Information Science & Technology. 2019, 9-12.
蔡凯杰. 基于D型光纤的表面等离子体共振折射率传感器研究 [D]. 南京: 南京信息工程大学. 2019, 9-12.
【6】Guo Z Y, Ge Y X, Shen L W, et al. Surface plasmon resonance fiber refractive index sensor based on MSM structure [J]. Semiconductor Optoelectronics. 2020, 41(2): 205-210.
郭志勇, 葛益娴, 沈令闻, 等. 基于MSM结构的表面等离子体共振光纤折射率传感器 [J]. 半导体光电. 2020, 41(2): 205-210.
【7】Li C H, Ma Z C, Hu X Y, et al. Preparation and application of microfluidic Raman detection chip [J]. Chinese Journal of Lasers. 2021, 48(1): 0100001.
李春赫, 马卓晨, 胡昕宇, 等. 微流控拉曼检测芯片的制备与应用 [J]. 中国激光. 2021, 48(1): 0100001.
【8】Xiao G L, Zhang K F, Yang H Y, et al. Refractive index sensor with double resonance peaks for D-type symmetric two-core photonic crystal fiber [J]. Acta Optica Sinica. 2020, 40(12): 1206001.
肖功利, 张开富, 杨宏艳, 等. D型对称双芯光子晶体光纤双谐振峰折射率传感器 [J]. 光学学报. 2020, 40(12): 1206001.
【9】Liu H, Bai B B, Zhang Y Z, et al. High-sensitivity temperature measurement based on SPR in gold-PDMS-coated photonic crystal fiber [J]. Chinese Journal of Lasers. 2020, 47(4): 0404003.
刘海, 白冰冰, 张砚曾, 等. 基于SPR效应的金-PDMS涂覆光子晶体光纤高灵敏度温度测量 [J]. 中国激光. 2020, 47(4): 0404003.
【10】Chen Q H, Han W Y, Kong X Y, et al. Detection of solution refractive index variation based on optical fiber surface plasmon resonance [J]. Chinese Journal of Lasers. 2020, 47(8): 0804003.
陈强华, 韩文远, 孔祥悦, 等. 基于光纤表面等离子体共振检测溶液折射率变化 [J]. 中国激光. 2020, 47(8): 0804003.
【11】Zheng Y B. Research on multi-core photonic crystal fiber laser and surface plasmon resonance sensor with photonic crystal fiber [D]. Tianjin: Tianjin University. 2012, 74-76.
郑一博. 多芯光子晶体光纤激光器及光子晶体光纤表面等离子体共振传感研究 [D]. 天津: 天津大学. 2012, 74-76.
【12】Wang F C. Study of outer coated photonic crystal fiber based surface plasmon resonance sensor [D]. Qinhuangdao: Yanshan University. 2019, 19-22.
王福成. 外层镀膜光子晶体光纤表面等离子体共振传感器的研究 [D]. 秦皇岛: 燕山大学. 2019, 19-22.
【13】Fan Z K, Zhang Z C, Wang B Z, et al. Research progress of photonic crystal fiber refractive index sensors based on surface plasmon resonance effect [J]. Laser & Optoelectronics Progress. 2019, 56(7): 070004.
范振凯, 张子超, 王保柱, 等. 基于表面等离子体共振效应的光子晶体光纤折射率传感器的研究进展 [J]. 激光与光电子学进展. 2019, 56(7): 070004.
Li Gangmin,Li Zhiyuan,Li Zhengran,Wang Jinmin,Xia Li,Yang Zhao,Wei Li. High-Sensitivity Optical-Fiber Microfluidic Chip Based on Surface Plasmon Resonance[J]. Chinese Journal of Lasers, 2021, 48(1): 0106002
李钢敏,李致远,李正冉,王锦民,夏历,杨曌,李微. 基于表面等离子体共振的高灵敏度光纤微流控芯片[J]. 中国激光, 2021, 48(1): 0106002